This invention relates generally to multi-energy x-ray machines and in particular to an improved method of discriminating between x-ray energies in such machines that permit lower x-ray flux rates and longer x-ray tube life.
Measuring the x-ray attenuation of an object at two different x-ray energies can reveal the composition of that object as a proportion of two arbitrarily selected basis materials. In the medical area, the basis materials may be "bone" and "soft tissue" allowing x-ray images to yield quantitative information about in vivo bone density for the diagnosis of osteoporosis. Alternatively, the basis "fat" and "lean" tissue may be selected to provide the indication of total body fat useful in the treatment of obesity or conversely wasting diseases.
Basis materials of "explosive" and "nonexplosive" materials may be used in the baggage scanning industry to augment images of the contents of baggage with the indication of the composition of the imaged contents.
Other basis materials may be selected for other industrial applications.
Referring to FIG. 1, a commercially available multi-energy x-ray machine 10, in this case a bone densitometer, includes an x-ray source 12 supported at one end of a C-arm 14 positioned beneath a patient support table 16. An energy sensitive detector 18 is held by the other end of the C-arm 14 opposite the x-ray source receives a fan beam 20 of x-rays from the x-ray source 12. The fan beam is formed by a collimator (not shown) being one or more x-ray opaque shutters which block x-rays in all but narrow rectangular cross section as is well understood in the art.
A patient (not shown) positioned on the patient support table 16 may be scanned by motion of the C-arm with respect to the patient so that the x-ray fan beam 20 illuminates the patient over a region of interest.
Referring now to FIG. 2, the x-ray source 12 may be a conventional polychromatic x-ray tube producing x-rays having a single mode spectrum 22 encompassing both high and low energy x-rays. The x-ray fan beam 20 is received by a k-edge filter 24 such as a cerium filter having an a real density of approximately 343 mg/cm.sup.2. The effect of the k-edge filter 24 is to preferentially block mid-energy x-rays to produce a bi-modal spectrum 26 having two peaks in regions of high and low energy.
The x-ray fan beam 20 continues through a patient 28 to arrive at an energy sensitive detector 18. Absorption of x-rays by the patient 28 produces at the energy sensitive detector 18 an attenuated bi-modal spectrum 30 also exhibiting the two peaks of bi-modal spectrum 26 but with lower amplitude.
The energy sensitive detector 18 may include a single scanned detector element or a number of detector elements 32 arranged in a linear or a real array. The detector elements in combination with motion of the C-arm (shown in FIG. 1) allow a spatial mapping of x-ray signals to particular lines through the patient 28 and thus imaging of the patient 28 and spatially dependent measurements of the patient 28 such as area densities.
A detector is "energy sensitive" as used herein if it can distinguish the fluence of x-rays at different energies. A number of energy discriminating detector types are known in the art including scintillation-type detectors in which the x-rays are converted to light via a scintillator material. The amount of light for each event indicates the energy of the x-ray photon. The scintillation material may be followed by a photo multiplier tube to amplify the light output and the light may be measured by any of a number of light detectors including but not limited to Charge-Coupled Devices (CCD). Ionization detectors which work by measuring current formed by an ionized gas in the path of the x-rays can provide energy discrimination through measurement of the amount of current generated at each photon event. Solid state detectors using photodiodes can provide energy discrimination through the use of filters in a stacked or side-by-side configuration. Cadmium Zinc Telluride (CZT) provides direct electrical outputs in response to detected x-rays in the form of pulses for each incident photon where pulse amplitude or area is proportional to the photon's energy.
As shown in FIG. 2, the output of the energy sensitive detector 18 for one detector element 32 may be a series of pulses 34 of varying times and heights corresponding to arrival times of related x-ray photons and the energies of those photons. The statistical distribution of the heights of the pulses 34 will conform generally to the attenuated bi-modal spectrum 30.
The signals for each detector element 32 may be received by an amplifier/pulse shaping circuit 35 and then by energy discriminator 36 (only one shown for clarity). The energy discriminator compares each pulse's height to a reference band 38 (implemented by a high and low voltage) which establish a high and low end point of an energy range for a plurality of window comparators 40(a) through 40(c). Generally, only pulses having heights within the corresponding reference band 38 will be passed by the window comparators 40 (i.e., pulse voltage peaks greater than the low voltage and lesser than the high voltage). Window comparators 40 can be constructed by two standard comparators, the first connected to the low references voltage and the pulse signal to provide a low output unless the pulse is above the low reference voltage and the second connected to the high references voltage and the pulse signal to provide a low output unless the pulse is below the high reference voltage. The outputs of the comparators are then logically ANDed together.
Referring now to FIGS. 2 and 4, each reference band 38(a)-(c) generally establishes a different detection zone in the attenuated bi-modal spectrum 30. Reference band 38(c) in conjunction with window comparator 40(c) defines a low energy (LE) range causing the detection of only x-ray photons in the lower peak of attenuated bi-modal spectrum 30. Similarly, ranges 38(a) and 38(b) together, establish with their window comparators 40(a) and 40(b), a high-energy (HE) range detecting photons in the higher energy peak of attenuated bi-modal spectrum 30. Within the HE range, reference band 38(b) further establishes a lower range (LR) and reference band 38(a) establishes an upper range (UR) equally dividing the HE range. The purpose of these subranges will be described below.
Each window comparator 40(a) through 40(c) is followed by an integrator 42 such as a counter which counts the total number of pulses passed by the comparator within the respective ranges LE and HE and subranges LR and UR of range HE.
The output of the integrators 42 is provided to a basis material processor 46 acting on high and low energy attenuation information to establish the composition of the intervening material of the patient 28. The low energy attenuation information is provided directly by the output of the integrator associated with window comparator 40c of the LE range whereas the LR and UR images are added together to form the high energy attenuation information of the HE range. The latter addition is shown by summing block 44.
The basis material processor 46 operates according to well known techniques to process the high and low energy attenuation information to determine a basis material decomposition such as may be displayed to an operator on an interface terminal 48 of conventional design. The basis material processor 46 may be a microprocessor-based computer of a type well known in the art.
Referring now to FIG. 4, variations in the signal chain between the detector elements 32 and the integrators 42 can cause the attenuated bi-modal spectrum 30 to vary in time by a compression or dilation along the horizontal or energy axis as indicated by attenuated bi-modal spectrum 30'. For accurate and repeatable measurements, it is therefore necessary to adjust the HE region to conform with changing location of the high-energy peak of the attenuated bi-modal spectrum 30'. This process of adjusting the location of the HE region provides an automatic gain control and is implemented through use of the lower region LR and upper region UR described above.
Referring now to FIG. 5, the automatic gain control function is implemented continuously or at periodic intervals during the acquisition of data from the energy sensitive detector 18. As indicated by process block 50 in a first step of this process, the received flux in the LR and UR regions are measured being generally the area under the attenuated bi-modal spectrum 30 or 30' within the ranges 38(b) and 38(a), respectively. If the LR flux is greater than the UR flux (plus a deadband E value representing an acceptable error tolerance) such as would be the case, for example, with the attenuated bi-modal spectrum 30 shown in FIG. 4, then this is detected at decision block 52 and at process block 54 the HE region (and LR and UR regions) are moved downward in energy by a predetermined increment.
Contrary-wise, if at decision block 55 it is detected that the LR flux is less than the UR flux (minus a deadband P, value), then the HE region is moved upward in energy (together with the LR and UR regions) per process block 57.
In this way, the HE region is centered on the high-energy peak of the attenuated bi-modal spectrum 30 regardless of the gain of the amplifier and pulse shaping circuitry 35.
In an alternative embodiment, producing the equivalent result, the gain of the amplifier/pulse shaping circuit 35 may be adjusted (along the dotted line path between the range adjuster 56 and the amplifier pulse shaping circuit 35) effectively stretching or shortening the attenuated bi-modal spectrum 30 on the horizontal axis and thus shifting the peak 62 with respect to the LR and UR regions.
Referring to FIG. 2, the process of changing the location of the HE region is performed by a range adjuster 56 which receives the UR and LR value and establishes the ranges 38(a) through 38(c) according to the flow chart of FIG. 5. The range adjuster may be discrete circuitry or may be a program operating on a separate microprocessor or the same microprocessor used for the basis material processor 46.
Referring now to FIG. 6, the comparison of the flux LR and UR regions and process of FIG. 5 serve to position the HE region about a local maximum of the high energy peak of the attenuated bi-modal spectrum 30 or 30'.
Referring again to FIG. 2, the x-ray tube serving as x-ray source 12 will typically be operated at high currents resulting in elevated x-ray tube operating temperatures and a reduced x-ray tube life. This need for high current operation or high tube "loading" results from a number of factors including the low absolute efficiency of x-ray tubes, the high degree of collimation of the x-rays into an x-ray fan beam 20 comporting with the area of the energy sensitive detector 18, the absorption of flux by the k-edge filter 24 and the need for a certain amount of flux for statistical accuracy the measurements being made.
It would be desirable to produce a multi-energy x-ray machine providing improved tube loading that would permit longer tube life.